High intensity lamp containing thermal shorting fuse

ABSTRACT

A high intensity lamp comprising an arc tube containing metal vapor such as sodium and mercury mounted within an evacuated outer jacket. A self-shorting arcing fuse is provided to forestall a destructive power arc should there occur a leak of air into the jacket or should the arc tube fail resulting in a low pressure of gas in the jacket. The fuse comprises a thermally deformable bimetal strip located to define the closest spatial approach between opposite current conductors within the jacket. A spot of brazing metal on the bimetal assists in welding the parts together at closure.

mite States Arendash tet [191 51 June 5,1973

8/1960 Breeding et al. ..3l5/75 12/1965 Gustin ..3l3/25 [57] ABSTRACT Ahigh intensity lamp comprising an arc tube containing metal vapor suchas sodium and mercury mounted within an evacuated outer jacket. Aself-shorting arcing fuse is provided to forestall a destructive powerarc should there occur a leak of air into the jacket or should the arctube fail resulting in a low pressure of gas in the jacket. The fusecomprises a thermally deformable bimetal strip located to define theclosest spatial approach between opposite current conductors within thejacket. A spot of brazing metal on the bimetal assists in welding theparts together at closure.

6 Claims, 3 Drawing Figures HIGH INTENSITY LAMP CONTAINING THERMALSHORTING FUSE BACKGROUND OF THE INVENTION The invention relates to highintensity vapor arc lamps comprising an inner envelope mounted within avacuum outer envelope and is particularly useful with high intensitysodium vapor lamps of the kind described in US. Pat. No. 3,248,590Schmidt, entitled High Pressure Sodium Vapor Lamp. In these lamps, anelongated inner envelope of ceramic materialsuch as a high densitypolycrystalline alumina tube contains a filling comprising an amalgam ofsodium and mercury and a rare gas to facilitate starting. Each end ofthe alumina tube is sealed by a refractory closure member, suitably aniobium end cap which serves as an electrical inlead and supports anelectrode internally. The ceramic arc tube is supported within an outervitreous envelope or jacket having at one end a screw base providingterminals to which the end caps of the arc tube are connected. Thejacket is evacuated in order to conserve heat and maintain the cold spotor lowest temperature portion of the arc tube at a sufficiently hightemperature.

The high pressure sodium vapor lamp has a relatively high startingvoltage due in part to the small diameter of the arc tube and the use ofxenon as a starting gas. Also the metal end cap used for the arc tubeclosures does not lend itself conveniently to the incorporation of anauxiliary starting electrode. To start and operate the lamp, a ballastcircuit is generally used which supplies a high voltage pulse,customarily near the peak of the open circuit waveform, until the lampignites. The pulsing circuit is disabled by the reduction in voltage atthe lamp terminals from the open circuit value to the lamp runningvalue, and the pulse is discontinued until the next time the lamp isstarted. When the lamp fails, the ballast continues to supply highvoltage pulses until power is disconnected or the lamp is replaced. Thelamp socket and the fixture wiring must have insulation capable ofwithstanding the high voltage pulses from the ballast over extendedperiods of time.

A serious problem encountered with high pressure sodium vapor lamps,particularly in the larger sizes such as lamps of 700 watts and 1,000watts rating or higher, is the possibility of a destructive failureresulting from uncontrolled arcing and the attendant hazards. Incopending application Ser. No. 230,761, filed Mar. 1, 1972 by Byron R.Collins, Juris Sulcs and Charles I. McVey, entitled High Intensity LampContaining Internal Shorting Fuse and assigned to the same assignee asthe present invention, two situations are described which may cause adestructive power arc to occur in the outer jacket notwithstanding thevacuum therein. The first situation is that wherein the vacuum outerjacket develops a slow air leak, for instance as a result of mishandlingor heat shock. As the internal pressure rises from a high vacuumcondition to -20 torr, the breakdown voltage across the stem leadswithin the jacket will drop to a few hundred volts. The ballast opencircuit voltage (480 volts in a 1,000 watt lamp) or the high voltagepulses supplied by the ballast circuit will insure ignition of adischarge which will become a destructive power arc as the bulb airs upfurther. The second situation is that wherein the arc tube suddenlyfails as when an end cap seal develops a leak. The release of sodium andmercury vapor and the arc tube starting gas into the jacket may cause anarc to start between the metal parts about the stem. Even though thesodium and mercuryvapor rapidly condense on the relatively cooljacketwalls and their residual pressure in the much larger volume of the outerjacket would not support an arc, the are once started can maintainitself in the vapor of the metal parts such as iron or nickel inleadsand supports to which it attaches.

In said copending application of Collins et al., a selfshorting arcingfuse is provided within the jacket to extinguish any arc occurring andsafely end the life of the lamp. The fuse structure comprises metalconductors defining an arc gap at the closest spatial approach be tweenopposite current conductors within the jacket, such that under the heatof the power are the metal conductors will deform and bridge over orshort circuit the gap. In one fusing arrangement, a metallic loopportion of one conductor encircles a portion of the oppo site conductorin close proximity, the loop is of nickel that will soften and flowwithout excessive oxidation, so that upon fusing together with theopposite conductor, a direct short is formed which extinguishes the arc.

SUMMARY OF THE INVENTION While the self-shorting arcing fuse of saidcopending Collins et al. application will operate reliably to extinguish the arc and safely end the life of the lamp, it tends to be slowacting and requires several fabricated parts which must be carefullyassembled. A simpler selfshorting arcing fuse which will close morerapidly and in a reliable and positive fashion to provide a lowresistance short circuit is desirable.

In accordance with my invention, I provide an improved self-shortingarcing fuse which comprises a thermally deformable bimetal stripdefining part of the arc gap. The arc gap is located at the closestspatial approach between opposite current conductors within the jacket.When a discharge starts, the bimetal is heated and starts to close evenwhile the discharge is still in the glow stage and without waiting forthe arc stage. Before the gap is completely closed, an arc dischargewill occur which heats up the proximate metal surfaces. When the gap isclosed, the hot metal fuses and the contacting parts remain attached.The permanent closing of the bimetal may be assisted by providing a spotof nickel coating in the area of contact which will serve to weld theparts together at the instant of closure.

DESCRIPTION OF DRAWINGS In the drawings:

FIG. 1 is a front view of a high pressure sodium vapor lamp comprising abimetal shorting fuse embodying the invention.

FIG. 2 is a fragmentary view of the neck end of the lamp turned relativeto the view of FIG. 1 and showing the fuse.

FIG. 3 is a sectional view through the lamp taken at plane 33 lookingtowards the base end in the direction indicated by the arrows, andshowing the fuse arced over and welded closed.

DETAILED DESCRIPTION Referring to the drawing and more particularly toFIG. 1, there is shown a high pressure sodium vapor lamp 1 comprising anouter tubular vitreous envelope or jacket 2. A central portion of thelamp has been cut out to shorten the figure and facilitate illustration.The jacket is make of a high temperature glass such as borosilicateglass andv is relatively thick-walled in order to withstand atmosphericpressure. The neck 3 of the jacket is closed by a re-entrant stem 4terminated in a press 5 through which extend heavy inleads or currentconductors 6, 7 which are connected respectively to the threaded shell 8and insulated center contact 9 of a conventional screw base. The base ismechanically retained on the neck of the jacket by screwing shell 8 overthreaded retaining ring 10 which has inwardly turned notches 11 whichengage corresponding dimples in the glass of the neck.

The inner envelope or are tube 15 is made of sintered high densitypolycrystalline alumina ceramic per U.S. Pat. No. 3,026,210 Coble,Transparent Alumina and Method of Preparation, or of otherlighttransmitting ceramic capable of attack the attach of sodium vaporat high temperatures. The tube is closed by thimble-like niobium metalend caps 16, 17 having skirt portions which fit around the ends of thetube. The end caps are sealed to the alumina by means of a sealingcomposition comprising a major proportion of aluminum oxide and calciumoxide and a minor proportion of magnesium oxide.

Thermionic electrodes are mounted in the ends of the arc tube andsupported from the end caps. Lower electrode 19 is shown in the drawingand comprises a tungsten wire coil or helix 20 wound around a tungstemshank or core 21 fastened in the end of a niobium tube 22 welded throughthe end cap. The electrodes are activated by metal oxides retained inthe interstices between turns of the coil, a preferred material beingdibarium calcium tungstate Ba CaWo as described and claimed incop'ending application Ser. No. 97,907 filed Dec. 14, 1970 by William E.Smyser et al., entitled Discharge Lamp Thermionic Cathode ContainingEmission Material," and assigned to the same assignee as this invention.Lower niobium tube 22 has an opening into the ceramic arc tube whichserves as an exhaust port during manufacture. The ionizable fillingconsisting of an inert gas, preferably xenon at a cold filling pressureof about 20 torr, and a sodium-mercury amalgam is introduced throughexhaust tube 22 which is then hermetically pinched off by a cold weldindicated at 24. Upper niobium tube 23 has no opening into the arc tubeand may be used to contain a small quantity of yttrium metal serving asa getter. In operation of the lamp, lower tube 22 is the coolest portionof the arc tube and serves as a reservoir for excess sodium-mercuryamalgam. The illustrated lamp is intended for base-up operation; in abase-down version of the same lamp, the arc tube is reversed relative tothe outer jacket and longer exhaust tube 22 is located at the base endso that it is again lowermost in operation.

The are tube is supported within the jacket by a mount comprising a siderod 26 made of two wires spot-welded together for greater stiffness,welded to inlead 6 close by the press and extending the length of theenvelope. Leaf spring members 27 at the stem end and 28, 29 at the domeend bear against the tubular jacketto maintain rod 26 in place. Aluminaceramic has a substantial coefficient of expansion and the range oftemperature traversed by the arc tube exceeds 1,000C. To allow forexpansion, only the lower exhaust tube 22 is completely restrained,being welded between straps 31 bridged across laterally extendingportion 32 of the side rod. The upper tube 23 extends freely through anencircling opening formed between straps 33 extending from side rod 26to short support rod 34 which is welded to inlead 7 and braced by leafspring 35. Ceramic sleeve 36 electrically insulates straps 33 from siderod 26. The electrical connection to end cap 16 of the arc tube isprovided by flexible strap 37 welded thereto. The space within outerjacket 2 is evacuated to a high vacuum in order to reduce heat loss fromthe arc tube. Air is pumped out through exhaust tube 38 prior to tip-offand a suitable getter such as barium may then be flashed within thejacket to assure a high vacuum.

In accordance with my invention I provide a selfshorting arcing fusewithin the jacket and close by the stem where the inleads 6, 7 emergeinto the jacket or interenvelope space. As shown in FIGS. 1 and 2, ashort length of bimetal strip 41 is fastened to inlead 6 close belowpress 5 in such fashion that it extends towards inlead 7. I prefer toattach the strip to side rod 26 by means of spot welds as indicated,rather than directly to inlead 6. This permits mounting the strip flatover the right angle bend in the side rod and at the same time its freeend is spaced away from inlead 7 by the thickness of side rod 26. Theresulting gap of about 0.1 inch at room temperature defines the closestspatial approach between opposite current conductors within the jacketfor this example wherein bimetal strip 41 may be 0.010 inch thick, 0.125inch wide and 0.75 inch long. The high expansion side of the bimetalstrip may consist of an alloy of 25% Ni, 8.5% Cr and the balance Fe; thelow expansion side, of an alloy of 50% Ni and the balance Fe; the twosides are equal in thickness.

In operation of the lamp, heat radiated by the arc tube and conducted byside rod 26 heats up bimetal strip 41 and causes it to bend and reducethe arc gap, but this initial deflection is insufficient to close thegap. However should an air leak occur or should a defective arc tubeseal release metal vapor into the outer envelope, a glow, and ultimatelyan arc, develops in the gap and the bimetal is strongly heated. Thebimetal flexes sharply and forcefully closes the gap whereupon its freeend fuses or welds to inlead 7 and the lamp is permanently shortcircuited, as shown in FIG. 3. To assist in welding the fuse closed, asmall piece or tab of brazing metal may be attached to the bimetalstrip. As illustrated, a short piece of nickel wire 42 is welded at theend of bimetal strip 41 to lie transverse to inlead 7 as shown. Nickelpiece 42 is softened by the arc and serves as brazing matrial to form afillet which securely attaches strip 41 to inlead 7 in the event ofclosure, assuring that the fuse members will not break apart uponcooling. Other suitable brazing materials which may be used for tab 42are copper, nickel-copper alloys, nickel-iron-chrome alloys, andstainless steel alloys.

A second self-shorting arcing fuse may be provided in the base of thelamp in order to extinguish any arc which might form therein. This outerarcing fuse may be similar to that described and claimed in U.S. Pat.No. 2,950,417 Breeding et al., Series Electric Lamp, Aug. 1960, and islocated within the stem tube of the jacket and enclosed within baseshell 8 so that it is in outside air. inleads 6, 7 have solid wire innerportions of nickel within the outer jacket, tungsten portions where theypass through the press 5, and external portions 6a, 7a of flexiblestranded nickel-plated copper wire wrapped around a solid nickel corewire. Lead tions 6a, 7a to each other. Glass sleeve 43 is made of 10high lead content glass which becomes relatively conductive when heated.Upon being touched by the arc, sleeve 43 will melt and permit aconductive juncture to be formed between wires 6a, 7a which will shortcircuit the arc. v

The bimetal fuse construction described has been tested and foundreliable but modifications thereof may of course be utilized and itslocation or orientation within the outer jacket may be changed forconvenience.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A vacuum jacketed high intensity vapor arc lamp comprising:

an evacuated vitreous outer envelope having a pair of inleads sealedtherein;

an inner elongated envelope containing ionizable vapor and havingelectrodes sealed into its ends;

said inner envelope requiring a starting voltage high enough to start anarc discharge in said outer envelope should the vacuum therein bedestroyed;

means supporting said inner envelope within said outer envelope andincluding conductors connecting said electrodes to said inleads;

and a short-circuiting arcing fuse in said outer envelope comprising abimetal conductor connected to one inlead and defining a gap at itsclosest spatial approach to an opposite current conductor connected tothe other inlead so that any arc forming in said outer envelope willoccur at said gap; said bimetal conductor being disposed to flex andclose said gap upon being heated by a discharge.

2. A lamp as in claim 1 wherein a small piece of brazing metal isprovided at the free end of said bimetal conductor for fusing to saidopposite conductor upon engagement thereof at occurrence of arcing.

3. A lamp as in claim 1, wherein said inleads are a pair of stiff wiressealed into said outer envelope through a press; I

said bimetal conductor is attached to one of said inleads close to saidpress;

and the free end of said bimetal conductor is spaced close to said otherinlead at the operating temperature of said lamp to define said gap andflexes to engage said other inlead when heated by an arc occurring insaid gap.

4. A lamp as in claim 1 wherein said inleads are stiff wires sealed intosaid outer envelope through a press;

said bimetal conductor is attached to one of said inleads close to thepress;

the free end of said bimetal conductor is spaced close to said otherinlead at the operating tempeature of said lamp to define said gap, andflexes to engage said other inlead when heated by a discharge occurringabout said gap;

and a small piece of brazing metal is provided at the free end of saidbimetal conductor for fusing to said other inlead upon engagementthereof by said bimetal at occurrence of arcing in said gap.

5. A lamp as in claim 4 wherein said brazing metal is selected from thegroup consisting of nickel, copper, nickel-copper alloys,nickel-iron-chrome alloys and stainless steel alloys.

6. A lamp as in claim 4 wherein said brazing metal is a short length ofnickel wire welded on said bimetal conductor and lying thereon in anorientation to en-

1. A vacuum jacketed high intensity vapor arc lamp comprising: anevacuated vitreous outer envelope having a pair of inleads sealedtherein; an inner elongated envelope containing ionizable vapor andhaving electrodes sealed into its ends; said inner envelope requiring astarting voltage high enough to start an arc discharge in said outerenvelope should the vacuum therein be destroyed; means supporting saidinner envelope within said outer envelope and including conductorsconnecting said electrodes to said inleads; and a short-circuitingarcing fuse in said outer envelope comprising a bimetal conductorconnected to one inlead and defining a gap at its closest spatialapproach to an opposite current conductor connected to the other inleadso that any arc forming in said outer envelope will occur at said gap;said bimetal conductor being disposed to flex and close said gap uponbeing heated by a discharge.
 2. A lamp as in claim 1 wherein a smallpiece of brazing metal is provided at the free end of said bimetalconductor for fusing to said opposite conductor upon engagement thereofat occurrence of arcing.
 3. A lamp as in claim 1, wherein said inleadsare a pair of stiff wires sealed into said outer envelope through apress; said bimetal conductor is attached to one of said inleads closeto said press; and the free end of said bimetal conductor is spacedclose to said other inlead at the operating temperature of said lamp todefine said gap and flexes to engage said other inlead when heated by anarc occurring in said gap.
 4. A lamp as in claim 1 wherein said inleadsare stiff wires sealed into said outer envelope through a press; saidbimetal conductor is attached to one of said inleads close to the press;the free end of said bimetal conductor is spaced close to said otherinlead at the operating tempeature of said lamp to define said gap, andflexes to engage said other inlead when heated by a discharge occurringabout said gap; and a small piece of brazing metal is provided at thefree end of said bimetal conductor for fusing to said other inlead uponengagement thereof by said bimetal at occurrence of arcing in said gap.5. A lamp as in claim 4 wherein said brazing metal is selected from thegroup consisting of nickel, copper, nickel-copper alloys,nickel-iron-chrome alloys and stainless steel alloys.
 6. A lamp as inclaim 4 wherein said brazing metal is a short length of nickel wirewelded on said bimetal conductor and lying thereon in an orientation toengage said other inlead transversely.